A standard (IEEE 802.11e) has been proposed for medium access control (MAC) quality of service (QoS) enhancements in wireless local area networks (LANs) and metropolitan area networks (MANs). The proposed standard is intended to enhance the 802.11 MAC to improve and manage QoS, expand support for applications with QoS requirements, provide classes of service, and consider efficiency enhancements in the area of the Distributed Coordination Function (DCF) and Point Coordination Function (PCF).
The proposed standard includes enhanced distributed channel access (EDCA), which defines the classes of service. FIG. 1 shows a block diagram of a conventional EDCA handler 10 within a wireless network station. EDCA handler 10 has four queues 12, 14, 16 and 18, corresponding to different access categories AC0, AC1, AC2 and AC3. Each access category (“AC”) corresponds to a different class of service, and each class of service has a different priority for transmission over the network. Generally, these classes of service include, in order, “best effort” (or highest priority), video, voice, and background, which all refer to types of network “traffic” (i.e., data and/or information transmitted in, on or over a network). EDCA handler 10 generally further includes logic for identifying the class of service for a particular block of data or information (not shown).
Using arbitration control circuits 22, 24, 26 and 28, the queues 12, 14, 16 and 18 contend (or arbitrate) amongst themselves for the opportunity to access the channel to which EDCA handler 10 is coupled. Virtual Collision Handler (VCH) 30 determines which of queues 12, 14, 16 and 18 may contend for access to the channel based on a number of parameters, including arbitration interference spacing AIFS [n] and backoff period BO[n], each of which may be independent for each queue. Generally, the higher the priority, the shorter the arbitration interference spacing and/or backoff period for a given queue.
FIG. 2 shows a block diagram of a channel arbitration scheme 200, including a plurality of EDCA handlers 60, 62 and 64 arbitrating through medium access control (MAC) access handler 70 for control of channel 80. MAC access handler 70 uses a conventional arbitration protocol to determine which of EDCA handlers 60, 62 and 64 gains control of (and thus transmits over) channel 80. DCF refers to the process of arbitrating and/or contending for access to channel 80. Referring back to FIG. 1, selection of a queue 12, 14, 16 or 18 that will contend for access to channel 80 may be referred to as virtual DCF.
Enhanced DCF (EDCF) refers to an improved process for differentiating between classes of service based on (i) arbitration interference spacing and (ii) minimum and maximum contention windows (CWmin[n] and CWmax[n]). FIG. 3 shows an exemplary virtual EDCF scheme 100 for differentiating between different classes of service in a wireless network station. After a data packet 110 has been transmitted over the channel by a station that has been granted access to the channel, receipt of packet 110 is acknowledged by the receiving station over the channel using a conventional ACK signal. The ACK signal is typically sent after a period of time corresponding to the shortest possible interference spacing in the network, designated by “SIFS” in FIG. 3.
Referring back to FIGS. 1-2, access to the VCH 30 (and subsequently, access to the MAC access handler 70 and channel 80) is granted based on the class of service. Referring again to FIG. 3, blocks 130, 132 and 134 represent data having high, medium and low priority transmission categories, respectively. Each of blocks 130, 132 and 134 can access the VCH only after a predetermined period of time following the ACK signal. This predetermined period of time is the sum of the arbitration interference spacing, or AIFS, and the “backoff” (or wait) period. The AIFS and backoff period differ for each class of service (or transmission category, referred to as “TC” in FIG. 3, a term which is substantially equivalent to “access category” or “AC”), and each TC cannot begin to backoff until its AIFS has expired.
In the scheme of FIG. 3, the AIFS for each TC is determined according to the following formula:AIFS[TC]=SIFS+(AIFSN[TC]*Tslot)  [1]where SIFS is as defined above, Tslot is the slot time, and AIFSN[TC] represents a fixed integer corresponding to the inverse order of priority (i.e., highest priority has the lowest AIFSN value). For the 802.11e specification, AIFSN has a minimum value of 2. PIFS, or PCF interframe spacing, is generally equal to (SIFS+Tslot). Generally, PIFS is shorter than DIFS but longer than SIFS. In PCF mode, PIFS is provided for Beacon packets to differentiate them (giving them higher priority) from the regular data packets that get transmitted at least after DIFS.
For example, in the scheme of FIG. 3, high priority TC 130 has an AIFSN value of 2, medium priority TC 132 has an AIFSN value of 3, and low priority TC 134 has an AIFSN value of 6. Thus, high priority TC 130 has the minimum AIFS, which is also known as the DIFS, or DCF interference spacing (also see, e.g., IEEE standard 802.11b). Contention window 140 is the period of time during which each TC must backoff, or wait, before any station may access the VCF/MAC access handler and transmit data over the channel (typically by transmitting a Request To Send [RTS] signal, then receiving a Clear To Send [CTS] signal from the receiving station). After each successful transmission, contention window 140 is reset to its minimum value, but if a transmission is unsuccessful, contention window 140 is incremented to its next successive value (up to its maximum value) until a transmission is successful.
Access is generally granted to the first TC to contend for access (e.g., transmit RTS). For a particular AC, the backoff period (after the applicable AIFS period) is calculated by selecting a random number from within the minimum contention window [0,CWmin[AC]] and multiplying that random number by Tslot. The window from which the random number is selected is inversely proportional to priority (i.e., the higher the TC priority, the smaller the window). Thus, since high priority TC 130 will generally be the first to contend for access to the channel (statistically speaking), generally it will be granted access when it is present. On a statistical basis, medium priority TC 132 generally will not contend for access to the channel until some number of time slots following high priority TC 130. Thus, medium priority TC 132 will generally be granted access only when there is no high priority TC 130 present to contend for access, although it is possible that, on occasion, the backoff period for medium priority TC 132 could be smaller than that of higher priority TC 130 because the number of time slots for a backoff period are picked randomly. Low priority TC 134 generally and/or statistically does not contend for access to the channel until some number of time slots following medium priority TC 132, so it will generally be granted access only when there is no high or medium priority block present to contend for access. In one implementation (an 802.11a PHY), SIFS is about 16 is and Tslot is about 9 μs. Thus, in that implementation, PIFS is typically about 25 μs, DIFS is typically about 34 μs, AIFS[medium priority TC] is typically about 43 μs, and AIFS[low priority TC] is typically about 70 μs.
To overcome potential contention problems from hidden nodes (see, e.g., the discussion below regarding hidden nodes), high priority TC 130 accesses the channel after transmitting an RTS signal to the intended receiver and receiving a CTS signal from the intended receiver. Terminals (nodes) in the “listening range” of the transmitter or receiver set their Network Allocation Vector (NAV) accordingly. However, the set of nodes that can hear the transmitter are not necessarily identical to the set of nodes that can hear the receiver. Those terminals that hear only the transmitter can be considered “hidden” from those terminals that hear only the receiver. Hence, the RTS/CTS handshake provides an extra protection mechanism in the case of hidden nodes.
Over time, proposals for EDCF have been made to the proposed 802.11e standard. It is generally believed that the ultimate standard, once approved, will include EDCF.
One such proposal is exemplified by the diagram in FIG. 4, which shows an EDCF scheme 150 that includes contention free bursting (CFB). In FIG. 4, “bursting” is exemplified by transmitting successive data packets 160, 162 and 164 from a particular access category in a station over a channel during a period of time designated as a transmission opportunity, or “TXOP.” An AC may transmit as many packets as possible during TXOP; however, if there is insufficient time remaining in TXOP to transmit another data packet, the AC generally relinquishes control of the channel. The transmission of subsequent data packets follows the usual process; i.e., the terminal, before initiating the transmission, waits for a length of time corresponding to its interference spacing and backs off. Each AC is generally assigned a unique TXOP length; the higher the AC priority, the longer the TXOP. After each data packet 160, 162 and 164 is transmitted, an acknowledge signal Ack-1 170, Ack-2 172 and Ack-3 174 is respectively transmitted in response. In CFB, TXOP and the data packets 160, 162 and 164 are configured or designed such that multiple data packets can be transmitted during a single TXOP. The same station maintains control of and/or access to the channel, and no arbitration or contention is possible. As a result, the period of time between a given acknowledge signal and the next data packet transmission can be as short as SIFS. In one implementation, CFB improved the throughput of a wireless network to about 50 Mbps (from about 35 Mbps in an otherwise identical wireless network not employing CFB).
Furthermore, in the example of FIG. 4, after each ACK signal is received, the transmitting station checks the time remaining in TXOP and determines whether another data packet can be transmitted in the remaining time. If there is sufficient time, another packet is transmitted; if there is insufficient time, the station relinquishes control of the medium and contends again. Since there is no data transmission at the end of TXOP, no station has control of the channel, and the highest priority AC contending for access to the channel will be granted access a predetermined period of time following the final acknowledge signal 174. This predetermined period of time is generally AIFS[AC] plus a backoff period 180, where AIFS[AC] is the arbitration interference spacing for the AC contending for access, and the backoff period is as defined above.
However, there are conditions in IEEE 802.11e-compliant wireless networks where one may transmit a number of data packets under CFB, but not receive or detect an acknowledge signal. For example, in the case of hidden nodes, an ACK signal may collide with a data transmission (or other signal occupying the channel), causing the transmitting station not to receive the ACK. Also, during periods of relatively high noise in the channel (e.g., when the signal-to-noise ratio [SNR] is relatively low), both data and ACK transmissions may be difficult and/or impossible to receive and/or process correctly. Such problems may result in a packet transmission error, requiring a retransmission of the packet. High packet error rates (PERs) result in effective loss of throughput in the network, which can be catastrophic and/or fatal for high priority traffic, such as HDTV transmissions. At this time, the present inventors are unaware of any techniques for avoiding packet transmission errors in an IEEE 802.11e-compliant wireless network using CFB.
A need therefore exists to implement EDCF with contention free bursting, but without unnecessarily and/or inadvertently introducing sources of contention and/or failure. Such an implementation in wireless networks will help to keep up with ever-increasing demands for increased wireless network speeds, bandwidth, throughput and flexibility.